U.S. patent number 7,424,093 [Application Number 11/805,665] was granted by the patent office on 2008-09-09 for fluorescent x-ray analysis apparatus.
This patent grant is currently assigned to SII NanoTechnology Inc.. Invention is credited to Takayuki Fukai, Yoshiki Matoba, Masanori Takahashi.
United States Patent |
7,424,093 |
Fukai , et al. |
September 9, 2008 |
Fluorescent x-ray analysis apparatus
Abstract
To provide a fluorescent X-ray analysis apparatus, whereby a
peak-back ratio is improved by effectively exciting a focused
element and a detection limit of the focused element is improved by
decreasing a scattered X-ray to be a background. A sample housing
has one or more wall surfaces made of a material through which an
X-ray transmits and an X-ray source is arranged so that a primary
X-ray is irradiated on the wall surface. In addition, the sample
housing is arranged so that a wall surface different from a wall
surface on which the primary X-ray is irradiated is opposed to an
X-ray detector incident window. Further, the primary X-ray from the
X-ray source is arranged so as to be able to irradiate the wall
surface of the sample housing to which the X-ray detector incident
window is opposed. The sample housing has a shape extending in
response to extension of a viewing filed that a detection element
in the X-ray detector is seen from the X-ray detector incident
window. In addition, on the wall of the sample housing, a metal for
secondarily exciting the focused element is arranged on an area
other than an area through which the primary X-ray transmits and an
area where the fluorescent X-ray from the focused element passes to
the detector.
Inventors: |
Fukai; Takayuki (Chiba,
JP), Matoba; Yoshiki (Chiba, JP),
Takahashi; Masanori (Chiba, JP) |
Assignee: |
SII NanoTechnology Inc. (Chiba,
JP)
|
Family
ID: |
38749488 |
Appl.
No.: |
11/805,665 |
Filed: |
May 24, 2007 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20070274441 A1 |
Nov 29, 2007 |
|
Foreign Application Priority Data
|
|
|
|
|
May 26, 2006 [JP] |
|
|
2006-146093 |
|
Current U.S.
Class: |
378/44;
378/208 |
Current CPC
Class: |
G01N
23/223 (20130101); G01N 2223/639 (20130101); G01N
2223/618 (20130101); G01N 2223/635 (20130101); G01N
2223/076 (20130101) |
Current International
Class: |
G01N
23/223 (20060101) |
Field of
Search: |
;378/44-50,57,58,66,68,79,208 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Song; Hoon
Attorney, Agent or Firm: Brinks Hofer Gilson & Lione
Claims
What is claimed is:
1. A fluorescent X-ray analysis apparatus comprising: a sample
container configured to store a quantity of sample and comprising
an end wall made at least in part X-ray transmissive and a side
wall tapered such that it becomes narrower towards the end wall; at
least one X-ray source located in close proximity to the side wall
to interrogate the quantity of sample with an X-ray through the
side wall; and at least one X-ray detector configured to detect,
through the end wall of the sample container, a fluorescent X-ray
excited out from the quantity of sample, wherein the at least one
X-ray detector comprises a periphery configured and placed so as to
generally coincide with a perimeter defined by an imaginary
extension of the tapered side wall from the end wall.
2. A fluorescent X-ray analysis apparatus according to claim 1,
wherein the at least one X-ray detector is located below a focal
point of the imaginary extension.
3. A fluorescent X-ray analysis apparatus according to claim 1,
wherein the side wall is made at least in part X-ray
transmissive.
4. A fluorescent X-ray analysis apparatus according to claim 1,
wherein the side wall has a circular cross section.
5. A fluorescent X-ray analysis apparatus according to claim 1,
wherein the side wall has a polygonal cross section.
6. A fluorescent X-ray analysis apparatus according to claim 1,
wherein the at least one X-ray source is arranged such that it
irradiates the end wall of the sample container with the X-ray.
7. A fluorescent X-ray analysis apparatus according to claim 1,
wherein the side wall comprises at least one primary filter
configured to pass through the X-ray within a different wavelength
range.
8. A fluorescent X-ray analysis apparatus according to claim 7,
wherein the at least one X-ray source is movable relative to the
sample container such that the at least one X-ray source irradiates
the quantity of sample selectively through the at least one primary
filter.
9. A fluorescent X-ray analysis apparatus according to claim 1,
wherein the end wall comprises at least one secondary filter
configured to pass though the X-ray within a different wavelength
range.
10. A fluorescent X-ray analysis apparatus according to claim 9,
wherein the end wall is movable relative to the at least one
secondary filter such that the at least one X-ray detector receives
the florescent X-ray selectively through the at least one secondary
filter.
11. A fluorescent X-ray analysis apparatus according to claim 1,
wherein the side wall is made at least in part of at least one
material which, excited by the X-ray, generates a different
secondary X-ray sufficient to excite the fluorescent X-ray out from
the quantity of sample.
Description
This application claims priority under 35 U.S.C. .sctn.119 to
Japanese Patent Application No. JP2006-146093 filed May 26, 2006,
the entire content of which is hereby incorporated by
reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a fluorescent X-ray analysis
apparatus for carrying out element analysis and composition
analysis of a measured sample by irradiating a primary X-ray on the
measured sample, inducing the fluorescent X-ray from the measured
sample, and measuring energy of this fluorescent X-ray and
intensity of the X-ray.
2. Description of the Related Art
A conventional general fluorescent X-ray analysis apparatus will be
described with reference to FIG. 10. Across a horizontal measured
sample base 1003, a measured sample 1005 is arranged upward of the
measured sample base 1003 and an X-ray source 1001, a primary
filter 1002, and a detector 1007 are arranged downward of the
measured sample base 1003. A reference numeral 1004 denotes a
primary X-ray irradiated from the X-ray source 1001, and a
reference numeral 1006 denotes a fluorescent X-ray generated when
the measured sample 1005 is exited by the primary X-ray 1004. In
this way, conventionally, the irradiated surface of the primary
X-ray on the surface of the measured sample and the surface opposed
to the detector on the surface of the measured sample are on the
same plane.
In addition, it is general that a detection efficiency of the
fluorescent X-ray from the focused element is improved by
approaching the detector and the X-ray source to the measured
sample as much as possible.
In addition, the apparatus having a primary filter in order to
improve a ratio between a peak intensity of the fluorescent X-ray
of the focused element and a background intensity mainly based on a
scattered X-ray or the like (hereinafter, referred to as a
peak-back ratio); the apparatus using a secondary target; and the
apparatus using an optical device for making the X-ray into
monochrome and focusing the X-rays are provided, however, all of
them have the structure such that the detector is opposed to a
point where the primary X-ray is irradiated (for example, refer to
JP-A-2004-150990 (P.3 and FIG. 1)).
In the conventional fluorescent X-ray analysis, it is general to
improve a peak-back ratio by using the primary filter when
confirming a presence of a trace heavy metal such as cadmium
contained in a light element major component which is composed of
C, O, and H or the like and a density thereof. The present method
is very useful, however, the primary X-ray is attenuated by
inserting the primary filter and as a result, the intensity that
the fluorescent X-ray of the trace heavy metal exited by the
measured sample enters the detector is low.
Therefore, in order to make the intensity of the X-ray entering the
detector stronger, the structure of approaching the detector and
the X-ray source to the measured sample is applied. However, since
both of the detector and the X-ray source are arranged so as to be
opposed to the same plane on the surface of the measured sample,
there is a limitation in a distance for approaching due to
interruption of the both structural objects when approaching them
to the measured sample. Therefore, it is general that a detection
limit is several wt pap in measurement of several hundreds seconds
when measuring a trace heavy metal in a light element.
In order to improve the detection limit of the trace heavy metal, a
peak-back ratio is an important factor, however, volume of the
intensity of the obtainable X-ray, namely, the sensitivity thereof
is also an important factor. Hereinafter, a general formula of the
detection limit is described. When the intensity of the X-ray is
increased, in proportion to this, the background intensity and
sensitivity are increased. In other words, from the following
formula, the detection limit is improved (lowered) in inverse
proportion to a root of the obtained intensity of the X-ray.
.times..sigma..times..times..sigma..times..times..times.
##EQU00001##
Here, DL denotes a Detection Limit, n.sub.p denotes peak intensity,
n.sub.BG denotes background intensity, TLT denotes Live Time, and
C.sub.i denotes density of a focused element.
SUMMARY OF THE INVENTION
The invention has been made taking the foregoing problems into
consideration and an object thereof is to provide a fluorescent
X-ray analysis apparatus, which efficiently excites a fluorescent
X-ray of a focused element so as to prevent the intensity of the
X-ray obtained by a detector from being lowered, effectively
improve a peak-back ratio, and improves a detection limit.
In order to attain the above-described object, to increase a
fluorescent the intensity of the X-ray from the focused element and
to decrease a background which becomes a noise are considered.
Therefore, at first, a fluorescent X-ray analysis apparatus
according to the invention may comprise a sample sealing member for
housing a solid sample or a liquid sample made of a material
through which an X-ray transmits; an X-ray source for generating a
primary X-ray entering from the side wall of the sample sealing
member in a radial pattern for X-irradiating the sample; and a
detector, which is disposed being opposed to a bottom face of the
sample sealing member and has an incident solid angle extending
from an incident point to a detection element in a direction of a
sample, for detecting a fluorescent X-ray to be generated from the
sample given the primary X-ray; and the fluorescent X-ray analysis
apparatus carries out analysis of an element of the sample from a
spectrum of the detected fluorescent X-ray. As described above, in
the conventional general fluorescent X-ray analysis apparatus, the
irradiated surface of the primary X-ray on the surface of the
measured sample and the surface seen by the detector on the surface
of the measured sample are on the same plane, however, the
invention is characterized in that the irradiated surface of the
primary X-ray on the surface of the measured sample and the surface
seen by the detector on the surface of the measured sample are
different. Thus, the X-ray source can be more firmly attached to
the sample sealing member, the primary X-ray can be put into the
sample sealing member by making a radiation solid angle larger, and
the primary X-ray can be irradiated to more samples. In addition,
the detector can be more firmly attached to the sample as same as
the X-ray source, so that an incident solid angle can be made
larger and more fluorescent X-rays can be put in the sample sealing
member. Therefore, the invention contributes to improvement of the
detection limit. Here, the X-ray source other than the X-ray tube
is also available if it irradiates a light in a radial pattern from
the side of the sample sealing member toward the measured sample.
For example, the secondary target and the optical device for making
the X-ray into monochrome may be also available.
In addition, the sample sealing member has a taper shape which
tapers off to the detector. Further, the taper shape is defined to
extend from the bottom face of the sample sealing member in a
direction opposed to the side where the detector is disposed in
response to extension of the visual field of the detector. Thereby,
the background which becomes the noise can be decreased and this is
based on the following reason(s).
The background is formed with the primary X-ray to be irradiated
from the X-ray source being scattered. A ratio that the primary
X-ray of the light element is scattered is higher. In a relation
between the X-ray energy and the intensity of the obtained X-ray
(hereinafter, referred to as an energy spectrum), with the
scattered X-ray having the same energy as the fluorescent energy
from the focused element entering the detector, an adverse affect
is given to detection of the focused element as a noise. Even when
the fluorescent X-ray of the focused element is generated, the
measured sample disposed in the field outside of a viewing field of
the detector does not contribute to rising of the peak intensity of
the fluorescent X-ray of the focused element since the X-ray cannot
reach the detector. However, the measured sample disposed in this
area scatters the primary X-ray and contributes to rising of the
background. Thereby, only leaving the measured sample disposed in
the area inside of the viewing field of the detector, no measured
sample is left in the area outside of the viewing field of the
detector, and this enable to decrease the scattered X-ray
efficiently and to decrease the background without lowering the
intensity of the fluorescent X-ray of the focused element.
Therefore, the sample sealing member has a taper shape extending
from the bottom face opposed to the detector in a direction that
the detector is seeing in response to extension of the visual field
of the detector. Thereby, most of or all measured samples are
housed in the area inside of the visual field of the detector and
this makes it possible to decrease the background of the focused
element.
Further, a metal wall is disposed, which generates a fluorescent
X-ray best suitable for exciting the focused element on the side
wall other than an incident part of the primary X-ray in the sample
sealing member. Thereby, an excitation efficiency of the focused
element is raised, so that it is possible to improve the peak-back
ratio when the spectrum is obtained by the detector and to increase
the intensity of the fluorescent X-ray of the focused element.
Here, by making the shape of the sample sealing member into a taper
shape extending from the wall opposed to the detector in a
direction that the detector is seeing in response to extension of
the visual field of the detector, the metal wall fluorescent X-ray
as a noise generated from the metal surrounding the side wall of
the sample sealing member is prevented from directly entering the
detector. In addition, the metal wall has a convenient mechanism,
which forms a part of the sample sealing member and is changed by
rotation of the sample sealing member, so that space-saving of a
driving part of the metal wall can be realized and it is possible
to efficiently excite the focused element by approaching the metal
wall for generating the metal wall fluorescent X-ray to the
measured sample more.
On the other hand, selectively exciting the focused element between
the X-ray source and the measured sample, the primary filter for
lowering the background is mounted. Thereby, it is also possible to
improve the peak-back ratio when the detector obtains spectrum. The
primary filter has a convenient mechanism, which forms a part of
the sample sealing member and is changed by rotation of the sample
sealing member, so that space-saving of a driving part of the
primary filter can be realized and it is possible to efficiently
excite the focused element by approaching the X-ray source for
generating the primary X-ray to the measured sample more.
In addition, mounting a secondary filter for selectively
transmitting only the fluorescent X-ray from the focused element
between the measured sample and the detector, the peak-back ratio
when the spectrum is obtained by the detector can be improved and a
saturated condition of the detector due to large amounts of the
incident X-rays can be prevented. The secondary filter has a
convenient mechanism, which forms a part of the sample sealing
member and is changed by rotation of the sample sealing member, so
that space-saving of a driving part of the secondary filter can be
realized and it is possible to efficiently excite the focused
element by approaching the detector to the measured sample
more.
In addition, the sample sealing member can be made into a sample
housing which is detachable and is formed in the same shape.
By using the above-described sample sealing member, the intensity
of the X-ray from the focused element which can be obtained by the
detector is increased, the background is decreased, the focused
element contained in the light element is detected with a high
sensitivity, and the detection limit can be improved.
The invention has the following advantages.
At first, by irradiating the primary X-ray from the sample sealing
member to the measured sample in a radial pattern, the fluorescent
X-ray generated from the entire cubic area where the primary X-ray
irradiated area and the area seen by the detector are superimposed
with each other is allowed to enter the detector, and thereby, it
becomes possible to improve the obtained intensity of the
X-ray.
Further, since the X-ray source and the detector can approach the
sample sealing member without given the interference based on the
structure of the apparatus as compared to the conventional case,
generating the fluorescent X-ray from the sample with a high degree
of density, it becomes possible to put these fluorescent X-rays in
the detector with a broader angle. In addition, by making the
sample sealing member into a taper in consideration of the viewing
field of the detector, the background is decreased, namely, the
peak-back ratio is improved.
From above, it becomes possible to improve the detection limit of
the focused element. In addition, the measured time can be reduced
at a detection level which could be realized by the conventional
apparatus.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a pattern diagram of a part of a fluorescent X-ray
analysis apparatus according to the invention;
FIG. 2 is a pattern diagram with respect to a principle of the
invention;
FIG. 3 is a pattern diagram of a part of a fluorescent X-ray
analysis apparatus in the case that sample housing is formed by a
primary filter, a metal wall for secondary excitation, and a
secondary filter;
FIG. 4 is a pattern diagram of an X-ray energy spectrum from an
X-ray source;
FIG. 5 is a pattern diagram of an X-ray energy spectrum after
transmitting through the primary filter;
FIG. 6 is a pattern diagram of an X-ray energy spectrum after
transmitting through the secondary filter;
FIG. 7 is a pattern diagram of a sample housing, of which wall is
partially formed by a plurality of primary filters;
FIG. 8 is a pattern diagram of a sample housing, of which wall is
partially formed by a plurality of metals for secondary
excitation;
FIG. 9 is a pattern diagram of a sample housing, of which wall is
partially formed by a plurality of secondary filters; and
FIG. 10 is a pattern diagram of a conventional fluorescent C-ray
analysis apparatus.
DETAILED DESCRIPTION OF THE INVENTION
The embodiment(s) of the invention will be described with reference
to the drawings.
The invention is characterized in that an X-ray source is disposed
being opposed to a side wall of a sample sealing member, an X-ray
from the X-ray source emits from the side wall of the sample
sealing member in a radial pattern so as to irradiate the measured
sample, a detector for detecting a fluorescent X-ray generated from
the sample receiving a primary X-ray is disposed being opposed to a
bottom face of the sample sealing member, and an incident solid
angle extending from an incident point toward a detection element
in a direction of the sample.
Further, the invention is characterized in that a shape of the
sealing member is formed in a taper which is gradually extended in
response to extension of a viewing field of the detector.
This point will be described in detail with reference to FIG. 2.
FIG. 2 is a principle view showing the invention. A detector 209 is
formed by a detector incident window 208, a detector wall 203, and
a detection element 204 disposed inside of the detection wall. The
detector wall 203 is made of a material which does not transmit the
X-rays. A reference numeral 207 denotes a border line of a viewing
field that the detection element 204 is seen from a detector
incident window 208. Even if the measured sample 202 emits a
fluorescent X-ray 206 of the focused element, a measured sample 202
disposed in the area outside of the viewing field of the detection
element 204 through the detector wall 203 does not contribute to
rising of a peak of the fluorescent X-ray of the focused element in
an energy spectrum and the measured sample 202 disposed in the area
scatters the primary X-ray so as to only contribute to rising of a
background which becomes a noise because the X-ray does not reach
the detection element 204. On the contrary, a fluorescent X-ray 205
irradiated from a measure sample 201 disposed in the area inside of
the viewing field of the detection element 204 enters the detection
element 204. Making the shape of the sample sealing member into a
taper extending in response to extension of the viewing field
through the detector wall 203 and making this into the same shape
as the area inside of the viewing field of the detection element
204, only leaving the measured sample disposed in the area inside
of the viewing field of the detection element 204, no measured
sample is left in the area outside of the viewing field of the
detection element 204, and this enable to decrease the scattered
X-rays effectively and to decrease the background without lowering
the intensity of the fluorescent X-rays of the focused element.
FIG. 1 is a pattern diagram of an X-ray optical system of a
fluorescent X-ray analysis apparatus according to the invention and
shows a positional relation between a sample housing 101 as a
sample sealing member, an X-ray source 103, a detector incident
window 113, a detector wall 111, and a detection element 110
disposed inside of the detector wall. A detector 114 is formed by
the detector incident window 113, a detector wall 111, and a
detection element 110. The detector wall 111 is made by a material
which does not transmit the X-rays. In FIG. 1, a grained measured
sample 102 containing a focused element in a minute amount in FIG.
1 is filled in the sample housing 101, which is made of an organic
material through which the X-rays can transmit relatively easily
and a material such as aluminum, silicon, and magnesium. Here, by
applying crush and compression process to the measured sample 102
and filling it in the sample housing 101 evenly with a high
density, it is possible to generate the fluorescent X-ray
effectively from the measured sample 102. Then, the measured sample
102 is set in an analysis apparatus together with the sample
housing 101.
The invention is characterized in that the location where the
primary X-ray 104 from the X-ray source 103 is irradiated and the
location where the detector incident window 113 and the detection
element 110 are facing are different on the sample housing 101.
Therefore, approaching the X-ray source 103 and the detector
incident window 113 to the sample housing 101 without receiving the
interference of both of them, they can be firmly attached with each
other. In FIG. 1, the detector incident window 113 is firmly
attached to a bottom face 109 of the sample housing 101. In
addition, the primary X-ray 104 from the X-ray source 103 is
irradiated in the vicinity of the bottom face of the sample housing
101, to which the detector incident window 113 is firmly
attached.
The primary X-ray 104 entered from a wall surface 105 of the sample
housing 101 is irradiated to the measured sample 102. The focused
element in the measured sample is excited by the primary X-ray to
generate a fluorescent X-ray 106. A part of the fluorescent X-ray
106 generated from the focused element in a radial pattern
transmits through the measured sample 102 and enters the detection
element 110 through the detector incident window 113. A reference
numeral 112 denotes a border line of a viewing field that the
detection element 110 is seen from the detector incident window
113. In the detection element 110, the detector incident window 113
is firmly attached to the sample housing 101 and the distance
between the detection element 110 and the sample housing 101 is
made shorter, so that a solid angle from the measured sample 102
for the detection element 110 is made larger and the fluorescent
X-ray 106 from the focused element enters the detection element 110
efficiently. This enables to improve the sensitivity.
Here, the X-ray source 103 for generating the primary X-ray 104 to
be irradiated to the sample housing 101 other than the X-ray tube
is also available if it irradiates a light in a radial pattern from
the side of the sample sealing member toward the measured sample.
For example, the secondary target and the optical device for making
the X-rays into monochrome may be also available.
In addition, by mounting the primary filter 105 allowing the X-rays
to selectively transmit there through for effectively exiting the
focused element from the primary X-ray 104 on the wall surface
which the primary X-ray 104 generated from the X-ray source 103
enters in the wall surface of the sample housing 101, the peak-back
ratio of the focused element is improved when obtaining the X-rays
by means of the detection element 110 and the saturated condition
due to enormous amounts of the X-ray detection by the detection
element 110 can be prevented. In addition, one or more primary
filters are mounted in readiness for the case that there is a
plurality of focused elements and there is provided a mechanism
which can switch the primary filter 105 continuously by rotating
the sample housing 101 without a complex mechanism.
Specifically, the present embodiment will be described with
reference to FIG. 7. FIG. 7 shows the mode that a primary X-ray 703
enters a primary filter 702 from a part of the wall surface of a
sample housing 701. A reference numeral 704 denotes a detection
element disposed inside of a detector wall 705. One or more primary
filters 702 are mounted on the wall surface of the sample housing
701 and only by rotating the sample housing 701, it is possible to
change the primary filter. Thus, a space of a driving part of the
primary filter can be saved and the X-ray source can be approached
to the measured sample more so as to be able to excite the focused
element.
On the other hand, on the wall of the sample housing 101, a metal
wall for secondary excitation 107 for generating a fluorescent
X-ray best suitable for exciting the focused element on the side
wall other than an incident part of the primary X-ray 104. Most of
the primary X-rays 104 entered the sample housing 101 transmit
through the relatively light measured samples 102 without
interaction with each other. The primary X-rays 104 transmitted
through the measured samples 102 may excite the metal wall for
secondary excitation 107 and as a result, a metal wall fluorescent
X-ray 108 best suitable for exciting the focused element in a
minute amount is generated. This metal wall fluorescent X-ray 108
effectively excites the focused element in the measured sample 102
and improves the peak-back ratio of the focused element in the
energy spectrum when the X-rays are obtained by the detection
element 110.
Further, since the sample housing 101 has a taper shape similar to
the viewing field of the detection element 110, it is possible to
prevent the metal wall fluorescent X-ray 108 generated from the
metal wall for secondary excitation 107 from directly reaching the
detection element 110.
FIG. 8 shows the mode that a primary X-ray 803 transmits through a
measured sample in a sample housing 801 so as to excite a metal
wall 802 and generate a metal wall fluorescent-X ray 804. A
reference numeral 805 denotes a detection element disposed inside
of a detector wall 806. One or more metal walls 802 are mounted on
the sample housing 801 and the metal has a mechanism which can
change the metal wall 802 continuously by rotating the sample
housing 801 without a complex mechanism.
In addition, by mounting a secondary filter for allowing only the
fluorescent X-ray 106 from the focused element to selectively
transmit there through on the wall surface 109 being opposed to the
detector incident window 113 in the wall surface of the sample
housing 101, the peak-back ratio when the X-rays are obtained by
the detection element 110 can be improved and a saturated condition
due to enormous amounts of the incident X-rays can be
prevented.
FIG. 9 shows the mode that one or more secondary filters 902 are
mounted on the bottom face of a sample housing 901. A portion 903
is made of a material not allowing the X-rays to transmit there
through and only allows an X-ray 907 entered a portion 904 to be
transmitted to the outside of the sample housing 901. Then, the
X-ray 907 reaches a detection element 906 through a detector
incident window 905 and there is provided a mechanism which can
switch the secondary filter continuously by rotating the sample
housing 901 without a complex mechanism.
Even when there is not provided the sample housing 101 shown in
FIG. 1, by making the shape of the measured sample housing similar
to that of the sample housing 101 and filling the measured sample
in the measured sample housing, an advantage of the invention can
be realized.
Even when there is not provided any one or plurality of the first
filter 105, the metal wall for secondary excitation 107, and the
secondary filter arranged on the wall surface 109 of the sample
housing 101, it is possible to realize a part of the advantage
according to the invention.
A part of the external wall of the sample housing 101 is formed by
any one or plurality of the primary filter 105, the metal wall for
secondary excitation 107, and the secondary filter arranged on the
wall surface 109 of the sample housing 101, it is possible to allow
the X-ray source 103 and the detection element 110 to approach the
sample housing 101 the most. As a result, it is possible to
maximize the advantage of the invention.
The invention is characterized in that the sample housing 101 has a
taper shape gradually extending in response to extension of a
viewing field that the detection element 110 is seen from the
detector incident window 113.
Therefore, according to the present embodiment, the X-ray source
103 is arranged on the side wall of the sample housing 101 and the
detector incident window 113 is arranged on the bottom face wall of
the sample housing 101. However, if the condition that the location
where the primary X-ray 104 is irradiated on the wall face of the
sample housing 101 and the location where the detection element 110
is seen from the detector incident window 113 are different is met,
it may be also possible to arrange the X-ray source 103 on the
surface different from the side wall of the sample housing 101 or
it may be also possible to arrange the detector incident window 113
on the surface different from the bottom face.
Hereinafter, citing analysis of Cd (cadmium) in a cereal as an
example, an embodiment of the invention will be described. Here,
the X-ray tube as the X-ray source, the X-ray detector as the
detector, and the sample housing as the sample sealing member are
used. FIG. 3 shows the mode that a cereal 302 containing Cd is
filled in a sample housing 301 and an X-ray tube 303, and an X-ray
detector 314 formed by an X-ray detector incident window 313, an
X-ray detector wall 311, and an X-ray detection element 310 are
arranged for a sample housing 301. Here, the X-ray detector wall
311 is made of a material which does not transmits the X-rays.
An energy spectrum of a primary X-ray 304 to be radiated from the
X-ray tube 303 is as shown in FIG. 4. The energy spectrum of FIG. 4
is formed by a continuous X-ray 401 to be radiated from a target of
the X-ray tube and a property X-ray 402. The primary X-ray 304
enters from the external wall of the sample housing 301 to be
irradiated to the cereal 302. Here, in order to excite Cd in the
cereal 302, energy higher than an absorption end of Cd should be
irradiated. Therefore, an X-ray of a lower energy than the
absorption end (26.7 keV) of Cd is absorbed on the external wall
surface opposed to the X-ray tube 303 of the sample housing 301 and
a primary filter 305 allowing the higher energy than the absorption
end of Cd to transmit there through is mounted, and a quality of
radiation of the X-ray is changed so as to be able to efficiently
excite Cd.
A reference numeral 504 of FIG. 5 denotes an energy spectrum of the
primary X-ray 304 after transmitting through the primary filter 305
and a reference numeral 502 represents an X-ray transmission ratio
of the primary filter. A radiation spectrum 503 is generated by
exciting the focus element in the measured sample by means of the
primary X-ray. A reference numeral 501 denotes a spectrum of the
X-rays entered the X-ray detector after transmitting through the
primary filter. Due to the effect of the primary filter, the peak
of the focus element can be confirmed. Some of the primary X-rays
304 entered from the external wall of the sample housing 301 may
interact with the cereal 302 in the sample housing 301 and others
thereof may transmit through the primary filter 305 without
interacting with the cereal 302. Further, some of the primary
X-rays interacting with the cereal 302 may emit an X-ray which is
peculiar to a constitutional element of the cereal 302 in the
sample housing 301, namely, a fluorescent X-ray in a radial pattern
and others thereof may scatter the primary X-rays in a radial
pattern to emit the scattered X-rays.
In order to detect Cd in the cereal 302 with a high sensitivity and
improve (lower) the detection limit of Cd, it is necessary to
decrease the scattered X-rays to be a background noise as much as
possible and increase the net amount of the Cd fluorescent X-rays
from the cereal 302 by means of the X-ray detector. In order to
realize the above-described two points, the shape of the sample
housing 301 is determined as follows.
In other words, the sample housing 301 is formed in a taper shape,
which is the same shape as the viewing field of the X-ray detector,
namely, the area inside of the viewing field that an X-ray
detection element 310 is seen from an X-ray detector incident
window 313, and gradually expands in response to expansion of the
viewing field. A reference numeral 312 denotes a border line of the
viewing field that the X-ray detection element 310 is seen from an
X-ray detector incident window 313. A reason why the sample housing
301 is formed in a taper shape is that a fluorescent X-ray 306 from
the area outside of the viewing field of the X-ray detection
element 310 cannot directly enter the X-ray detection element 310
and it is only necessary for the cereal 302 to be located in the
area inside of the viewing field of the X-ray detection element 310
where the fluorescent X-rays of Cd directly enter the X-ray
detection element 310. Then, since the scattered X-ray as the
background noise is increased depending on the amount of the cereal
302, by forming the sample housing into a taper shape, it is
possible to increase the scattered X-ray to be a background noise
without providing the cereal 302 in the area outside of the viewing
field of the X-ray detection element 310 which does not contribute
to detection of the Cd fluorescent X-rays.
The X-ray tube 303 is arranged near the wall surface 305 of the
sample housing 301 or so as to be firmly attached to the wall
surface 305 of the sample housing 301. On the wall surface 305, the
primary filter is mounted.
In addition, on the wall surface other than the incident surface of
the primary X-ray 304 on the side wall surface of the sample
housing 301, a metal wall for secondary excitation 307 for emitting
the fluorescent X-rays having energy slightly higher than the
absorption end energy of Cd, for example, tellurium (Te) or the
like is mounted. Thereby, radiating fluorescent X-rays 308 from the
metal wall for secondary excitation 307 by the X-rays transmitting
through without interacting with the cereal 302 in the primary
X-rays 304 emitted to the sample housing 301 and secondarily
exciting Cd in the cereal 302, enable to increase the amounts of
the fluorescent X-rays of Cd. Then, by defining the sample housing
into a taper shape as described above, this also serves to prevent
the fluorescent X-rays 308 emitted from the metal wall for
secondary excitation 307 to be a background noise from directly
entering the X-ray detection element 310. In many cases, the metal
wall is arranged between the X-ray source and the measured sample
also in the conventional fluorescent X-rays, however, the invention
is characterized in that the measured sample is arranged between
the X-ray source and the metal wall.
The fluorescent X-rays and the scattered X-rays radiated from the
cereal 302 may emit the X-ray detection element 310. The X-ray
detection element 310 has a limitation in countable number per unit
time, so that when the X-rays more than the limitation number
enters, the number of the X-rays which can be counted in practice
is decreased and this deteriorates the detection efficiency.
Therefore, by mounting a secondary filter 309 such as Ag having an
absorption end of energy slightly higher than the fluorescent
X-rays of Cd on the wall surface opposed to an X-ray detector
incident window 313 on the wall surface of the sample housing 301,
the fluorescent X-rays of Cd are not absorbed much and by absorbing
the scattered X-rays having higher energy than the fluorescent
X-rays of Cd more, saturation of counting is prevented.
A reference numeral 604 of FIG. 6 shows the energy spectrum of the
fluorescent X-ray 306 of the scattered X-ray and the focused
element at the X-ray detection element 310 when the secondary
filter 309 is not used. By mounting the secondary filter, the
energy spectrum of the fluorescent X-ray 306 of the scattered X-ray
and the focused element after transmitting through the secondary
filter 309 is represented by 601. A reference numeral 602
represents an X-ray transmission ratio of the secondary filter.
Thus, decreasing the background around the focused element by means
of the secondary filter, it is possible to improve a ratio between
a peak 603 of the focused element and a background (a peak back
ratio).
* * * * *